Multi-input multi-output (MIMO) is known as an example of a technology realizing an increase in throughput of a wireless local area network (LAN). In MIMO, it is possible to improve a data rate (that is, frequency utilization efficiency) by sending and receiving data using many antennas on the transmitting side and the receiving side, and also it is possible to perform advanced beamforming. Beamforming is a technology that can give directivity to a transmission beam and/or a received beam and change the shape of the beam arbitrarily by controlling the amplitude and/or phase of at least one of the signal to be transmitted and the signal to be received, at each antenna.
In beamforming, the amplitude and phase of the signal to be received are controlled so that the maximum signal power is obtained at the receiving side, for example, after estimating on what kind of propagation path a signal sent by the transmitting side (for example, terminal) is received at the receiving side (for example, base station). In order for the receiving side to estimate the state of this propagation path, for example, channel state information CSI is used.
CSI indicates channel state information (that is, information indicating a state of a channel that is a propagation path of radio waves in wireless communication) and is measured at the receiving side (for example, base station) based on the quality information when the receiving side receives a reference symbol (RS, hereinafter sometimes referred to as “CSI-RS” or “SRS (Sounding RS)”) used to measure the channel state information sent from the transmitting side (for example, terminal). The receiving side (for example, base station) measures channel state information (CSI) indicating a state of a propagation path between the transmitting side and the receiving side based on the reference symbol transmitted from the transmitting side (for example, terminal). The receiving side calculates a channel estimation matrix whose components are transfer functions from each antenna on the transmitting side to each antenna on the receiving side based on the measured channel state information (CSI) and performs beamforming by using this channel estimation matrix. In this way, the receiving side (for example, base station) can perform wireless communication by MIMO with a single transmitting side (for example, terminal) and perform wireless communication (that is, MU (Multiple User)-MIMO spatial multiplexing transmission) by MIMO with a plurality of transmitting sides (for example, terminals).
As the related art relating to sending a data signal by using beamforming in a communication system to which MIMO is applied, for example, Patent Literature 1 has been proposed. In order to be able to transmit data signals by applying beamforming also to user terminals located away from the base station, Patent Literature 1 discloses that a downlink reference signal (for example, channel state measurement reference signal) is transmitted to a user terminal by beamforming using a fixed weight and an appropriate weight is set by using an estimation result from the user terminal.